Following detection of ventricular fibrillation, prior art implantable defibrillators typically deliver a defibrillation shock as soon as their high voltage (HV) capacitors are charged, and in more advanced systems, after ventricular fibrillation (VF) is reconfirmed. The timing of the shock is either not related to the status of the VF signal or is delivered synchronous with the next cardiac complex after reconfirmation and/or charge completion.
In "Genesis of Sigmoidal Dose-Response Curve During Defibrillation by Random Shock: A Theoretical Model Based on Experimental Evidence for a Vulnerable Window During Ventricular Fibrillation," in PACE 1990, 13:1326-1342, Hsia and Mahmud discuss a theoretical model and experimental results for using a surface electrocardiogram (ECG) to identify a window of VF during which the heart is more susceptible to defibrillation. In their experiments, they compared the VF waveform voltage at the time of shock delivery for successful and unsuccessful shocks. The shock strength was held constant, and was chosen to produce a defibrillation success rate of 50%. When using a recording from a lead II surface ECG, the absolute VF voltage (AVFV) was found to exhibit significantly larger values for successful defibrillation as compared to unsuccessful defibrillation. However, their experiments using intracardiac electrograms from two epicardial patch electrodes instead of a lead II ECG to track the state of VF showed no significant difference in AVFV between successful and unsuccessful defibrillation. They do not disclose how using the window of VF susceptible to defibrillation could be used in an implantable device.
In an abstract from an American Heart Association conference in Atlanta, November 1993, Hsia and Frerk et al. describe "On-line Electronic Identification of a Period of Vulnerability to Defibrillation using Real-Time Ventricular Fibrillation Waveform Analysis". If implemented as described, however, this system would not work in a conventional implantable device because the electrogram from a bipolar pair of endocardial pacing and sensing electrodes would not provide the information necessary to generate a useful AVFV, which is needed to determine the window of susceptibility to defibrillation.